In such shifting devices the rotary movement of the gearshift rod serves to pre-select the individual shift gates and does not generally require any large shifting forces. The axial, longitudinal movement of the shift rod serves to engage the gear desired and, particularly in the case of transmissions for heavy vehicles and vehicles in which the driving seat is some distance away from the transmission, requires larger shifting forces.
Present-day utility vehicles, such as omnibuses and trucks, with their forward control design, provide transmission filling space such that the transmission is necessarily far removed from the driving seat. The distance is particularly large in vehicles with under-floor or rear-mounted engines. Owing to the long and sometimes stiffly moving shift linkage in mechanically shifted transmissions, accurate gearshifts are often made more difficult.
To allow the driver of a motor vehicle to focus his attention completely on the road traffic, he must be assisted and supported as much as possible in all the activities required for driving the motor vehicle.
Every driver knows how decisively important it can be, to be able to operate the transmission without difficulty in difficult traffic situations. In this respect pneumatic shift aids for utility vehicles of any size can be helpful.
Previously known servo-shifting devices are partially built directly onto the transmission and comprise a control rod accessible from outside and piston rods. The shift linkage is connected to the control rod. The assistance mechanism is activated by longitudinal movement of the control rod. This type of control is found in combination with two rod or cable shifts. A disadvantage here is the sealing of the control and piston rods by bellows and the lack of lubrication. In trucks the point concerned is exposed to much dirt. When the shift linkage ratio is changed, the beginning of the servo-assistance also changes or it has to be adapted to the linkage ratio by modifying the valve. The same applies to shift aids consisting of a control valve and a servo-cylinder that are separate from one another. The valve and cylinder are connected, via ball joints, to the shift lever and a cantilever, which is in turn fixed on the transmission. This arrangement has the added disadvantage that during every shift the components move relative to the transmission and the vehicle's chassis, so the air lines by which the valve and the cylinder are connected to one another can be perforated by chafing.
Such pneumatic shift aids are also known in a divided configuration, consisting of a mechanical-pneumatic control portion and a separate, pneumatic force portion. A shift aid of divided configuration is known from Loomann; Zahnradgetriebe (Geared transmissions); 2nd edition; Springer Verlag; 1988; p. 225. The control portion is a mechanically actuated control valve, which is actuated by the shift linkage. In this case movement of the selection lever during gearshifts is transmitted mechanically directly to the transmission. When the shifting movement is transmitted, the control valve is actuated and at the same time the manual shifting force is transferred by a lever to the transmission. The manual shifting force is additionally assisted pneumatically by a compressed-air cylinder. This compressed-air cylinder, which is a two-position cylinder with an integrated hydraulic damper, forms the pneumatic force portion. In this case the manual shifting force is not reflected directly proportionally. The paths between the control and force portions are long, and the structure takes up considerable space. Damage of the compressed-air lines between the control and force portions cannot be avoided.
DE 195 39 472 A1 discloses a shifting device with a servo-assistance mechanism for a motor vehicle transmission. A control rod of the servo-assistance mechanism that can move axially within a piston rod is provided, and co-operates via a shift linkage, with a shift lever. On the piston rod is arranged a piston that can be acted upon on both sides by a pressure medium, and the piston rod co-operates with means for shifting the geared change-speed transmission. When the control rod is moved axially in the piston rod, control valves can be actuated by actuating pistons. The shifting force exerted, via the shift lever and shift linkage, on the control rod is transferred proportionally, via the piston rod, to the means for shifting the geared change-speed transmission. During this the two sides of the shifting device produce shifting forces of different size, so that different gear steps of the vehicle's transmission can be engaged with different shifting forces. The different shifting forces are enabled by different valve characteristics, produced by actuating piston areas of different size and/or by different geometrical dimensions of the two surfaces arranged on opposite sides of the piston.
From DE 10 2006 006 651 A1 a shifting device with a servo-assistance mechanism for a motor vehicle transmission has been made known, which comprises means for shifting and selecting a gear step of a transmission and a control rod of the servo-assistance mechanism, upon which a manual shifting force that is to be supported acts. Elastic elements are provided in the shifting device to change or limit the manual shifting force that acts on the servo-assistance mechanism before the force is applied to the servo-assistance unit.
DE 10 2006 006 652 A1 also discloses a shifting device with a servo-assistance mechanism for a motor vehicle transmission, which comprises means for shifting and selecting a gear step of a transmission and a control rod of the servo-assistance mechanism upon which a manual shifting force to be supported acts. Spring elements are provided in the shifting device to modify the manual shifting force applied to the servo-assistance mechanism within the servo-assistance mechanism before and/or during the production of the servo-assisting force.
To produce different servo-assisting forces, different compliances or elasticities are needed. Depending on the design this can be complicated to realize and can require structural space which is often not available.
DE 10 2004 042 609 A1 proposes performance characteristics for a servo-assistance mechanism of a shifting device of a motor vehicle transmission. As a function of a manual shifting force or a shift phase the characteristics have sections with different gradients or proportionality to the manual shifting force. DE 10 2004 042 609 A1 does not disclose how the characteristics can be realized by design means.